1. Field of the Invention
The invention relates to a device and a method for dividing or fragmenting semiconductor material. In particular, the invention relates to a method and device for fragmenting semiconductor material using electrodes made from the semiconductor material to be fragmented or reduced in size.
2. The Prior Art
High-purity semiconductor material is used to produce solar cells or electronic components, such as storage elements or microprocessors. The dopants that are introduced in a controlled manner are the only impurities that a material of this nature should contain. It is therefore desirable to keep the concentrations of harmful impurities as low as possible. It is often observed that even highly pure semiconductor material is re-contaminated during the course of further processing to give the end products. Thus, laborious purification steps are required again and again, in order to recover the original purity.
Atoms of foreign metals, which become incorporated into the crystal lattice of the semiconductor material, interfere with the charge distribution and may reduce the functioning of the subsequent component or cause it to fail. Consequently, contaminations of the semiconductor material by metallic impurities are especially hazardous. This applies especially to silicon, which is the most frequently used semiconductor material in the electronics industry. High-purity silicon is obtained by thermal decomposition of silicon compounds such as trichlorosilane, which are highly volatile and are therefore easily purified by distillation processes. In this process, the silicon is obtained as a polycrystalline material in the form of rods having typical diameters of from 70 to 300 mm and lengths of from 500 to 2500 mm.
A large proportion of the rods are used for the production of crucible-drawn single crystals, for strips and sheets or for the production of polycrystalline solar-cell base material. Since these products are produced from high-purity, molten silicon, it is necessary to melt solid silicon in crucibles. In order to make this process as effective as possible, large-volume, solid pieces of silicon, such as the polycrystalline rods mentioned above, must be cut up or fragmented prior to melting.
Conventionally, this always results in a superficial contamination of the semiconductor material, because the fragmentation is carried out with metal crushing tools, such as jaw crushers, rolling crushers, hammers or chisels.
When carrying out this fragmentation, careful attention should be paid to avoid contaminating the surfaces of the fragments with foreign materials. In particular, it is critical to avoid contamination from metal atoms, since these atoms can adversely alter the electrical properties of the semiconductor material. If the semiconductor material is fragmented using mechanical tools in the customary fashion, such as with steel crushers, then the fragments must subjected to a laborious and cost-intensive surface purification before being melted.
According to German Patent Application DE-38 11 091 A1 and its corresponding U.S. Pat. No. 4,871,117, it is possible to decompact solid, large-volume silicon bodies to accomplish fragmentation using tools whose operating surfaces are made of materials which do not entail contamination, or do so only to a minor extent, such as silicon, nitride ceramics or carbide ceramics. The decompacting is achieved by using heat to generate an external temperature gradient in the piece of silicon which is to be broken up and setting a surface temperature of from 400 to 1400.degree. C., and then reducing this temperature quickly by at least 300.degree. C., so that the temperature gradient is at least partially reversed.
In order to produce the temperature gradient, the solid material is introduced into a furnace and heated. However, this process has the drawback that foreign materials which have been adsorbed at the surface of the semiconductor material are set in motion and/or accelerated during the heating phase. In this way, the foreign materials pass from the surface into the crystal lattice of the semiconductor material, and hence evade cleaning measures, which are only able to remove impurities situated close to the surface. Moreover, it is virtually impossible in the abovementioned process to avoid contamination of the semiconductor material by foreign materials released from the furnace material during heating.